This invention is a miniature monolithic multiple capacitor that is useful in small electronic circuits and that is particularly adapted for use with hearing aids designed to be inserted in the external acoustic meatus or aural canal. The invention is also directed to a method of making the monolithic multiple capacitor. In particular, it is a method of and a means for providing a device that contains many or all of the capacitors necessary for a circuit, especially an analog circuit, that fits into a relatively small space such as the external aural canal of a human being.
The development of integrated circuits has made it possible to place many circuit elements in a single semiconductor chip. Where part or all of the circuit is an analog circuit, the capacitor or capacitors that are needed for the circuit are often difficult or impossible to achieve as elements of an integrated circuit. The result has usually been to add required capacitance as individual external capacitors that take up a significant amount of volume in the circuit. This is particularly a problem in hearing aids that are designed to be inserted in the external acoustic meatus or aural canal of the ear. Such hearing aids are attractive to users because the hearing aids are relatively invisible to the observer who is thus unaware that someone is using a hearing aid. There are also operating advantages to be gotten from putting a hearing aid in the aural canal to couple acoustically to the tympanum or eardrum. The average diameter of the external acoustic meatus, however, is small enough to present a considerable challenge to the designer of a hearing aid to fit there all required components, including a microphone, amplifier, speaker, battery, circuit assembly, and the like.
Monolithic multilayer single capacitors are known. They are typically formed in groups of hundreds or thousands by printing a layer of conducting ink on a sheet of green tape or greenware, which is a thin layer of a powdered ceramic dielectric material held together by a binder that is typically organic. Such sheets, typically although not necessarily of the order of five inches by five inches, can be stacked to form 800 or more monolithic ceramic capacitors. After thirty to one hundred or so such layers are stacked, they are compressed and cut apart into individual capacitors. Heating the compressed individual device according to a desired time-temperature profile then drives off the organic binder and sinters or fuses the powdered ceramic material into a monolithic structure. The printed layers are then connected electrically in a pattern that provides one or more parallel-plate capacitors. The external electrical connections are thus made after the compressed combination is sintered or fused. Proper selection of the dielectric constant and loss tangent of the ceramic material, the separation of the conducting layers from each other by the ceramic material, and the surface area of the conducting layers makes it possible to design a capacitor to a predetermined value of capacitance, voltage rating, and loss tangent.
An example of a structure similar to that described above is given in U.S. Pat. No. 4,661,884, entitled "Miniature, Multiple Layer, Side Mounting High Frequency Capacitor," issued Apr. 28, 1987. The '884 patent teaches a single capacitor in which external conductive terminals are brought out to be soldered to a circuit board. A somewhat similar example is given by U.S. Pat. No. 4,312,026, entitled "Chip Ceramic Capacitor," issued Jan. 19, 1982. The '026 patent teaches in one embodiment a plurality of conducting layers separated by a dielectric material and connected together alternately on opposite sides of the structure to form a capacitor with interleaved conducting plates. Both the '884 patent and the '026 patent teach structures that comprise individual capacitors and not multiple capacitors as disclosed and claimed here.
A structure containing multiple capacitors is taught by U.S. Pat. Nos. 4,419,714, entitled "Low Inductance Ceramic Capacitor and Method for its Making," issued Dec. 6, 1983, and 4,430,690, "Low Inductance MLC Capacitor with Metal Impregnation and Solder Bar Contact," issued Feb. 7, 1984. The abbreviation "MLC" is defined in the '690 patent to mean multilayered ceramic, formed by interspersing layers of green tape or greenware with electrical conductors. Green tape or greenware is a layer of powdered ceramic material held together in the form of a tape or other thin structure by an organic binder. After the green tape is compressed, heating it drives off the binder and sinters or fuses the ceramic material into a monolith.
Both the '714 patent and the '690 patent are directed to forming layers of parallel-plate capacitors with different interconnections. The '714 patent teaches external connections that are brought out to bus bars that can be connected externally to select desired values of capacitance for a particular circuit. The '690 patent teaches external connections that are designed to be soldered to a plurality of solder bars to mount the capacitors to a circuit board.
A hearing aid comprises a battery or other portable energy source, a microphone, an amplifier, and an output transducer or receiver. The amplifier may include some desired form of signal processing. In contrast to the capacitors described above, which are described as being mounted to circuit boards, dimensions in the human external aural canal are small enough that it is barely possible to fit an additional substrate or integrated circuit containing the required electronics, and the necessary external capacitance, into the canal. It would therefore be useful to have a capacitor or array of capacitors that can be bonded directly to the chip that contains the integrated circuit to make a compact package.
An additional problem exists with multilayer ceramic structures having a plurality of capacitors. This is the fact that there is stray or parasitic capacitance between electrical conductors that are not connected by conducting paths but that are separated by, or adjacent in, dielectric media. One way to reduce or control such stray capacitance is to place ground planes at appropriate locations. This generally has the effect of splitting a single parasitic capacitance into two larger series parasitic capacitances such that the net value of capacitive coupling is typically less than that of the single parasitic element without the ground plane. This is appropriate with capacitors that float electrically in circuit use; this occurs when neither side of the capacitor is to be grounded. However, each of these series capacitors provides a new stray capacitance that couples to the ground node, and this capacitance is typically larger than the original parasitic coupling between the conductive paths without the ground plane; it is usually undesirable. Whatever measures are taken, it is impossible to eliminate all stray capacitance between pairs of electrical conductors in a monolithic ceramic capacitor.
One of the elements of stray capacitance that is typically especially noticeable in a monolithic multiple ceramic capacitor is the capacitance between adjacent terminals. A common way to reduce or control this stray capacitance is to drill holes in the monolithic material, either before or after it is fired, to insert air, which has a relatively low permittivity, in the direct path between the terminals. This has the disadvantage of limiting the distance that the air goes into the material to a radius that is less that half the separation of the terminals, and thus of limiting the number of external connections that can be made along an edge by requiring that the external connections be separated by the radius of the drill.
It would be an advantage to accept the fact that parasitic capacitance will exist in a multi element device and to use some or all of the separate parasitic capacitances as circuit elements. This could be done by designing the circuit for desired values of parasitic capacitance, by designing a circuit to use the values of parasitic capacitance that exist as a result of the design of the main capacitance values, or by a combination of these.
It is an object of the present invention to provide a better monolithic multiple capacitor.
It is a further object of the present invention to provide a monolithic multiple capacitor with reduced or controlled capacitance between adjacent terminal tabs.
It is a further object of the present invention to provide a monolithic multiple capacitor in which stray capacitance between pairs of terminals is used to form useful circuit elements.
It is a further object of the present invention to provide a monolithic multiple capacitor in which stray capacitance between pairs of terminals is set to controlled values to form useful circuit elements.
It is a further object of the present invention to provide a monolithic multiple capacitor that is adapted to serve as a mount on the top, bottom, or both, for one or more integrated circuits or surface-mounted components with a lower amount of coupling through stray capacitance between adjacent capacitors.
It is a further object of the present invention to provide a monolithic multiple capacitor that is adapted for use as part of a hearing aid to be inserted in the external aural canal.
It is a further object of the present invention to control values of stray capacitance in a monolithic multiple ceramic capacitor to repeatable values by separating regions of the monolithic structure from other regions by a layer or layers of ceramic material having dielectric constants lower than the dielectric constant of the ceramic material used to form the capacitors in the device.
Other objects will become apparent in the course of a detailed description of the invention.